Apparatus and method of molding variable resistors



July 25, 1950 T. B. GIBBS ET AL APPARATUS AND METHOD OF MOLDING VARIABLE RESISTORS Filed Feb. 14, 1947 2 Sheets-Sheet 1 v v vvvvv v1 ATTY.

APPARATUS AND METHOD OF MOLDING VARIABLE RESISTORS Filed Feb. 14, 1947 Fig. 5

2 Sheets-Sheet 2 lllll f 39 SI/ 30 22 I4 h E? I {"l 32 J ss I r I l 28 INVENTORS THOMAS B. GIBBS GEORGE W. GILMAN GORDQN F LAING Patented July 25, 1950 UNITED "STATES PATENT OFFICE APPARATUS AND METHOD OF MOLDING VARIABLE EESISTORS 10 Claims.

" The present invention relates in general to apparatus and method of molding variable resistors, potentiometers, but more in particular -to variable resistors in whichtheu-resistance element has the form of a compound helix, and the :-object of'the invention is to produce anew and improved variable resistor of this character.

whereby the material is prevented from flowing tothe interior of the helix in the molding operationythus keepingthe resistance'wire clean on the inside of the helix where it is engaged by the wiper or contact member ofthe resistor. This feature is claimed in the instant application, which is a division of application Ser. No. 573,680, filed January 20, 1945, Patent No. 2,495,-

321, granted January 24', 1950.

The foregoing and other features of the invention will be described fully hereinafter, reference being had to the accompanying drawings, in which- Fig. l is a vertical section through a. variable resistor or potentiometer embodying the invention;

i Fig. 2 is a transverse section taken on the line 2-2, Fig. 1;

Fig. -3 is a transverse-section taken on the liine 3-3,-Fig. 1;

Fig. 4 is a view, partly in section; which shows the construction of the resistance element; 'and Fig, 5 is a sectionalvview of a suitable mold with which-the'process of molding thecasing may be carried out. Referring tothe: drawings;'the variable resistor or potentiometer thereinshown comprises a casing" I0, having an end closure or cap l l, a, resistance element F2 in the form of compound helix, and a rotatable slider or contact member I3 which isadapted 'for'travel along the resistance element to make contact with the successive turns thereof. Theresistanceelement I2 is providedwith terminals I4- and I5 and the slider or contact "member" It has a terminal l6. When the device is used as a potentiometer a source of 'electricatpotential isconnectedto the terminals I4 and l=5 and a variable potential is delivered at the terminall6 depending on the position of the slider l3, as iswell known? When used as a variable resistor connections are-made to terminals l6 and I4; or to terminals i6 and I5, and the slider i3 is rotatedin one direction or the other to vary the resistance between the connected terminals in knownlmanner.

The resistance element l2 comprises a core l1, Fig. 4, having an insulating coating" l8 of*-suitable insulating materiaLand ahelically wound resistance wire I9.' In-the device shown, the core is of No. 14 copper wire, and the resistance wire base diameter of'2.18mils." The resistance wire is woundon-the core in spaced turnsjwhile the core is straight, and theencls of the winding are secured by soldering a few turns together as indicated at' 2i]. This preventsthe coils'from unwinding.- The insulated'corewith the *resistance wire wound thereon is then formed into a helix by winding it on a suitable mandrel having a helical groove therein. In the winding operation the relatively softicopper core is given a permanent set and is removed'fromthe form by unscrewing it in the manner of removing -nut from a bolt.

As illustrated inFig. 1 of thedrawingsthe core has somewhat more than ten complete "turns and the terminals M-andl5"are"3600 degrees apart, measured in terms ofthe' rotation of'the slider or contact member 13. The number of turns in the-core helix may be varied, however, as also the size of the core and the size of the resistance wire, in order tovary' the ohmic resistance of the resistance element, or'for' other reasons," The totalohmic resistanceof the resistance element shown'and described is "about 20,000'ohms between terminals.

A'suitablemoldfor making the casing I0 is illustrated in Fig. 5 andcomprises the base plate 2l', the core 23, the mold 22, thecylinde'r Z5 and the piston 26. 'The mold is of the. extrusion type and its construction conforms generally to the construction of similar molds which are incommon use. It will not be-necessary therefore togive more than a. brief description of the several parts,

"The 0013 2311318 acylindrical' base 21' which is secured to thejbase plate 2| by meansf'of screws such a's28. 'The mold22 rests onthe base plate 2! and'has a'cavitywhich conforms to-theoutside dimensions ofthe' casing 10. "The mold is properly located on the base plateby m ans of dow l pins: Such s 2. Therese?! of the core- 23- closes'themol'd cavity at the bottom and the remainder of the core extends into the cavity where it defines the inside wall of the casing. It will be noted that the core includes a cylindrical section 30 which has a helical groove 3|. The upper end of section 30 is an annular inclined plane 43 the ends of which are joined by the vertical abutment 44. The refererence character 24 indicates an insert which is slidable in the mold 22 and which is attached to the base 21 of the core by means of dowel pins 32 and 33. This insert provides a support for the terminals l4 and I5.

'The cylinder closes the mold cavity at the top and has one or more gates such as 34 which lead from the interior of the cylinder to the mold cavity. The cylinder has a centrally disposed recess at the bottom for receiving the bearing member 35, which is held in place by the spring pressed pin 36. The piston 26 has a tapered slot 31 at the end.

It will be understood that means is provided for heating the mold, but since heating means suitable for this purpose is well known, the showing thereof has been omitted in order to avoid complicating the drawings.

The operation of molding a casing will now be described, it being assumed for this purpose that the several parts of the mold are disassembled.

As the first step in the operation a prepared .resistance element [2 in theform of a compound helix is taken and is screwed on to the grooved section of the core 23. The turns of the helix are slightly smaller in diameter than the section 30 and the core I! of the helix has sufficient resiliency to make the helix hug the core and follow the groove3l as it is screwed on. The groove has a depth equal to about one-half the diameter of the helix core I! and the turns of the groove are spaced apart so as to separate the turns of the helix and hold them in spaced relation as shown.

The terminal I5, which may be already prepared, has a flexible metal strip 38 soldered to it at one end. The free end of this strip is now soldered to the resistance wire on the upper turn of the helix at a point about an inch or so from the end. The strip 38 overlaps a number of turns of the resistance wire and all these turns are soldered together and to the strip. At this time the strip 38 is straight and together with the terminal 15 extends upward parallel to the axis of the core 23. The terminal i4 is arranged similar to terminal [5 and has a flexible metal strip .39 similar to strip 38 which is soldered to the resistance wire a short distance from the end of the last turn of the helix. As previously mentioned the helix has somewhat more than ten complete turns, preferably about ten and a quarterturns. .A suitable guide may be used to facilitate the correct angular. location of the terminals and strips when the latter are soldered in place. The terminals may be in vertical alignment, as indicated in Figs. 1 and 5, but if a full 3600 degree range is essential the terminals are spaced apart angularly by a few degrees, or enough to insure that the short-circuited turns of resistance wire at one end of the helix do not overlap those at the other end. Fig. 2 illustrates this in the case of terminal i4, which is angularly displaced to the left, it being understood that terminal I5 is similarly displaced to the right.

Having attached the terminals as described, the operator now applies a covering 40 to the grooved section 30 of the core and the helix l2 4 I supported thereon. This covering consists preferably of several layers of a suitable textile fabric such as linen which is thoroughly coated and impregnated with a heat setting plastic material. This material may be the same kind of material that is to be used for the casing. Bakelite has been used with good results. The covering may be applied by taking a strip of the coated and impregnated fabric cut on the bias, and wrapping it around the helix to form a cylindrical sleeve, then applying a few turns of silk thread and tieing it to hold the sleeve in place. Preferably, however, a number of such sleeves are prepared in advance by wrapping suitable strips of the coated and impregnated fabric on a mandrel and tieing them with thread. With a supply of such sleeves on hand, the operator covers the helix by taking one of the prepared sleeves and slipping it into position around the section 30 of the core, as shown in Fig. 5.

The operator now bends the metal strips 38 and 39 around the ends of sleeve 40 so as to bring the terminals into approximately the positions in which they are shown and then places the mold insert 24 in position on the core 23. To effect this operation the dowel pins 32 and 33 are started into the corresponding holes in the core base 21 and at the same time the terminals are started into the holes in the insert, after which the insert may be pressed into position against the core base. If the terminals l4 and I5 fail to fully enter the holes in the insert they may be pressed in with the aid of a pair of pliers or other instrument.

The operator now takes a couple of washers 4|, made of the same material as the sleeve 40, and places them in position on the end of the core as shown.

As mentioned before, suitable heating means is provided for the mold and parts thereof in cluding the core 23. The mold 22 and cylinder 25 may be kept hot, but it is convenient to cut off the supply of heat to the core 23 while the operations just described are being performed. The core will cool off somewhat, therefore, and will have to be re-heated. The heating means may be a hot plate, or an electricalheating element embedded in the base plate 2!, for instance. Whatever the means employed may be, the necessity for heating the core may be taken advantage of for partially curing the sleeve 4!! and the. washers 4|, which otherwise would require a separate operation. To attain this object a hood is placed around the core while it is being re-heated, and the heat being confined, the temperature inside the hood is raised sufliciently to bringabout the desired partial curing of the sleeve and washers, or rather of the Bakelite material with which they are impregnated.

The heated core may now be placed in position on the bed of the press and the mold 22 may be lowered into position as shown, being properly located with respect to the core by the dowel pins such as 29. The cylinder 25, with the bearing member 35 inserted therein, is then placed on the mold. The cylinder should be so oriented with reference to the mold and core that the gate 34 is not directly above the terminal l5, to avoid danger of breaking the strip 38 by the inflowing Bakelite material.

A preheated cake 42 of Bakelite may now be placed in the cylinder 25, after which pressure is applied by means of the piston 26 in the usual manner. vThe Bakeliteis reduced to a plastic condition and flows into the mold through the gates 34, filling the mold cavity andforming the desired casing. The heat and pressure should be maintained for a sufiicient length of time, usually about 3 minutes, to fully cure or set the material. When the plastic material. starts to flow into the mold cavity it first fills the: space above the washers 4 E but as soon as pressure begins to build up the washers are bent downward around their edges and the plastic material flows down to fill the rest of the mold cavity. These washers seal the end of the sleeve 40 and'prevent the entrance of the free flowing plastic material into the sleeve. As soon as the mold cavity becomes filled the pressure builds up to a high value and the heat and pressure softens the Bakelite material with which the washers and sleeve are impregnated. The washers and the sleevethus become bonded to the rest of the material and form a lining for the casing which under the great pressure applied is caused to conform exactly to the core and to that portion of the resistance element which is exposed outside of the groove 3!. Looking at it another way, the main turns of the resistance element and the individual turns of the resistance wire become embedded in the lined casing, which fills the spaces between the turns and the groove 3-H beyond the ends of the helix, whereby the resistance element becomes firmly fixed in the easing as it is formed. 7 1

After the casing has become cured and set the mold may be removed from the press and taken apart. The cylinder 25, with the piston 26 retained therein, is first separated from the mold 22. This may be accomplished by driving wedges between the cylinder and mold .to break off the sprues in the gates 34, after which the cylinder may be lifted off, the bearing member 35 remaining with the casing in which it is embedded. The sprues break where they have the smallest diameter, which is next .to the casing. The core 23 may now be removed from the mold by use of a fixture comprising a plate similarto the plate 2| having three pins therein corresponding to the dowel pins 29 but somewhat smaller in diameter. The mold is laid on its side and the fixture is applied to the open end with the pins in the holes occupied by the dowel pins. A few blows with a hammer will new drive the dowel pins and the core out of the mold. The insert 24 comes out with l the core, to which it is attached by the dowel pins 32 and 33. With the core out of the mold insert 25 may be pulled off and the completed casing may then be unscrewed from the core. When the piston 26 is removed from the cylinder 25 the residue of molding material, now formed into a Bakelite disc with. attached sprues, comes out with the piston from which it can. readily be detached by a blow with a hammer. The apparatus is now ready for molding another casing.

The process as described is. a hand molding process, but it will be understood that in practice known arrangements for manipulating the mold and parts thereof with the aid of the press will be employed. Each mold should be provided with several cores so that while one core isin the mold other cores may be in courseof preparation. Three cores may be used, forexample, in a three stage process, comprising an assembly stage, a core heating and sleeve curing stage, and a molding stage.

An essential feature of the process is the use of the sleeve til with the associated washers 4| which prevent the free flowing Bakelite from entering between the turnsof the resistance wire into the groove 3| of the core and coating the turns of resistance wire on the inside of the helix, where they are engaged by the slider l3. The Bakelite impregnated fabric of which the sleeve is composed readily enters between the turns of the core wire H as far as the core of the mold, but the spaces between the resistance wire turns in the groove 3| are very small and the fabric of the sleeve cannot enter them. These spaces are thus effectively sealed off by the sleeve and entrance of the Bakelite into the groove 3| is prevented. This result is promoted by the partial curing of the sleeve and washers prior to the actual molding operation.

Referring now to Fig. 1 again, after the casing It has been molded as described, it is placed in a lathe and the bearing member 55 is turned down to the proper dimensions, the hole for the shaft .45 is drilled, and the threads are cut for the nut 45. The casing and resistance element are now ready to be assembled with the other parts to complete the variable resistor or potentiometer.

The cap ll, like the casing H3, is a Bakelite molding and is of the proper size to fit into the open end of the casing, as shown in Figs. 1 and 3. The cap has a centrally disposed metal insert 41 which is drilled and finished after the molding operation is completed. This operation is carried out by any suitable and known process and need not be described.

V The terminal IS, on the outside of the cap II, is secured in place by means of a rivet 48. This rivet also serves to hold the contact spring 49, located on the inside of the cap, and conductively connects the terminal with the contact spring. The contact spring is bifurcated and carries the two contacts 55 and 5!, as shown in Fig. 3. Also shown in Fig. 3 is the stop 52 which is formed integrally with the cap. The stop has the same height as the wall on which it is formed.

The shaft 45 is rotatable in the bearing member 35 and has an axial bore at the left hand end to receive the cylindrical end 55 of the lead screw 55. At the other end the lead screw has a cylindrical section 5! which is located in the hole drilled in the metal insert t! of the cap I l. The lead screw is thussupported on the cap H at one end and on the shaft 45 at the other end.

Y The cylinder 58, preferably made of brass, is supported on the two discs 52 and 54, which are made of suitable insulating material such as micarta. The disc 52 is rotatable on the part 51 of the lead screw, while the disc 54 is rigidly fixed to the shaft 45. The lead screw 56 carries the nut 59 which includes a support 60 for the slider or contact spring l3. This support projects through the longitudinal slot 6| in the cylinder 58.

I The shaft 45, lead screw 55, and cylinder 58 are assembled outside the casing IE3. The disc 54 is first placed on the shaft against the flange 53 and is secured by staking. The cylinder 58 is then placed in position on the disc 54 and the end of the cylinder is turned in by .a spinning operation, which firmly secures the cylinder to the disc. The lead screw 56 with the nut 59 threaded thereon may now be introduced into the cylinder, with the end section 55 inserted in the bearing opening in shaft 35. The disc 62 is then placed on the section 51 0f the lead screw and is pressed into the end of cylinder 58. At this point the cylinder and shaft should be tried for end play on' the lead screw, holding the disc 52 against the internal shoulder formed in the cylinder 58. The lead screw has shoulders formed attheopposite ends of the threaded section, the end sections 51 and 55 being of reduced diameter, and one of these shoulders is adapted for engagement by the disc '62 and the other by the end of shaft 45. There should be a small amount of end play, sufficient to insure free rotation of the cylinder. To facilitate the manufacturing operations and to insure that another operation will not have to be performed on the lead screw to shorten the distance between the shoulders the parts are preferably so designed and proportioned that ordinarily there will be too much end play. Then during the assembling operation if too much end play is found the excess is taken up by means of one or more small washers placed on the lead screw atone end.

Having tested for end play and having made the necessary adjustment, if any was required, the operator inserts the disc 63 into the end of cylinder 58 on top of the disc 52 and then performs another spinning operation which turns in the end of the cylinder and secures it to the discs 62 and 63. The disc 53 is a contact member, adapted to be engaged by the contacts 50 and on contact spring 49, and is preferably made of silver, or is silver plated.

The contact spring I3 may now be mounted on the support 60 by means of a small screw as shown in Fig. 1. At one end the spring !3 is provided with a contact 64, Fig. 2, and at the other end it is formed into a loop 55 which extends through the slot 6| in cylinder 58 and out again. The slot is necessarily made somewhat wider than the support to insure that the support will be freely movable, but the play which would otherwise result from this construction is taken up by the spring, which holds the support against one side of the slot. The nut 59 and support 55! are made of suitable insulating material and may be of molded Bakelite, for example. No filler should be used.

The contact spring I3 is conductively connected to the cylinder by means of a flexible braided conductor 66 which has one end soldered to the contact spring l3 and the other end soldered to the cylinder 58. The conductor 66 should be attached to the cylinder at a point about midway between the ends of the cylinder and should be long enough to permit movement of the support 60 along the slot Bl as far as it can go in both directions but not so long that it can touch the resistance element when the support is centrally located in the slot and the conductor is slack.

When the shaft 45, lead screw 56, cylinder 58 and the associated parts have been assembled as described, they are ready for assembly in the casing 10. For this purpose the casing is sup-' ported in a fixture which holds it in a vertical position with the open end up. The shaft 45 is now rotated in a counter-clockwise direction relative to lead screw 56 until the support Bil comes to the end of the slot Si in cylinder 58 and is then rotated approximately 90 degrees in a clockwise direction. The cylinder assembly is then brought to'a position above the casing, with the shaft 45 extending downward, and is so oriented relative to the casing that the contact 64 on contact spring I3 is in alignment with the terminal i4. From this position the cylinder assembly is lowcred into the casing, the shaft 45 entering its bearing in the bearing member 35, until the movement is arrested by the end of shaft 45 engaging a stop with which the fixture is provided. The operator now inspects the contact 64 through the open end of the casing to make sure that it is properly centered on the end turn of the resistance element and adjusts the'stop up or down if necessary.

The cap H is now inserted in the end of the Casing it, with the lead screw projecting through the hole in the metal insert 41. The cap should be inserted with the terminal It a few degrees past terminal [4 in a clockwise direction to insure that the stop 52 will clear the support 60. The cap having been seated properly, the operator holds the lead screw with a screw driver and rotates the cap in a counter-clockwise direction far enough to bring the terminal it into alignment with terminals l4 and H5. The lead screw is now soldered to the insert 41 and the casing with the parts assembled therein is removed from the fixture.

The next operation is the adjustment of the stop 52. In order to carry out this adjustment an ohmmeter is connected between the terminals l6 and 14, the support 60 is held against the stop by applying a counter-clockwise torque to the shaft 45, and the stop is adjusted by rotating the cap H in one direction or the other until the contact 64 is positioned on the last turn of the resistance wire which is soldered to the strip 38, counting from the end of the resistance element. In this position of the contact the ohmmeter will read substantially zero and will show an increased reading immediately responsive to clockwise rotation of the shaft '45. When the adjustment is completed the cap is temporarily held in position by a strip of tape to avoid any danger of its rotating relative to the casing.

The device may now be inspected and tested that if no defects are found the holes are drilled for the drive screws such as It and the screws are inserted. The cap H is thus securely fixed in the casing.

The finished device may be mounted on a panel, such as H, as shown in Fig. 1. A knob may be mounted on the shaft 45 for rotating it, or the shaft may be rotated by gears, a flexible shaft, etc. depending on the particular situation in which the device is employed.

As mentioned before, the device is adapted to function either as a variable resistor or as a potentiometer. Assuming that it is to be used in the former capacity, the necessary circuit connections are made to the terminals 16 and M. The circuit through the variable resistor extends from terminal It by way of rivet 48, spring 49 and contacts 50 and 5| in parallel, disc 63, cylinder 58, conductor 66, slider or contact spring I3 and contact 64, resistance wire IQ of the resistance element, and strip 38 to terminal i4. With the support resting against the stop 52, which is the counter-clockwise stop, only about one-half turn or less of the resistance wire is included in the circuit and the resistance between the terminals is substantially zero.

When the shaft 45 is rotated in a clockwise direction, the cylinder 58 rotates the support 69 and nut 59 on the lead screw 56, causing the support 60 to travel to the right along the slot 6! in the cylinder. The threads of the lead screw have the same pitch as the helical turns of the core of the resistance element, so that as the support 60 is rotated the contact '64 travels along the resistance element and engages successive turns of the resistance wire. As the turns are cut into the circuit one after the other the resistance between the terminals l6 and I4 is proportionately increased, as will be understood.

When theshaft has been rotated somewhat less than 360 degrees the support 60 has moved far enough to the right so that it clears the stop .52 when it passes it. The rotation in a clockwise direction may continue for 3240 degrees more, or for a total of 3600 degrees, when the support 60 will engage the clockwise stop and the rotation will cease. Theclockwise .stop is not shown, but it will be understood that it is formed in the end of the casing Why the abutment M, Fig. 5. The contact '65 will'now be in engagement with a turn of the resistance wire which is adjacent the turns soldered to the strip 38 associated with terminal 1 5 and the resistance between terminals [6 and I l will be a maximum.

It will be noted that clockwise rotation of the shaft 35 increases the'resistance while counterclockwise rotation of the shaft decreases the resistance." For any particular angular setting or the shaft the resistance will always be the same regardless of whether thesetting is approached in a clockwise or counter-clockwise direction. This desirable result is due to the fact that there is no lost motion in the drive between the cylinder 58 and the support 60, the part 55 of the contact spring 53 being effective to hold the support against the side of the slot 6| in the cylinder.

The operation of thedevice as a potentiometer will be understood from what has already been said and need not be explained in detail.

While my improved method of molding has been described with reference to a specific article and with the use of specific materials it will be understood that this has been .done to facilitate the description and without intending to limit the invention. Other articles can be made and other types of plastic materials can be used in certain cases, materials known as thermo-plastics, for example. I do not therefore wish to be restricted to the exact form of the invention which is shown and described herein but desire to include and have protected by Letters Patent all forms and modifications thereof which come within the scope of the appended claims.

We claim:

1. The method of molding a casing around a resistance element having the form of a compound heliX, which consists in providing a mold having a helically grooved core, supporting said resistance element on said core, covering said resistance element and the grooved portion of the core with a layer of textile fabric impregnated with a partially cured heat setting plastic material, inserting the core in the mold, and injecting heat setting plastic material into the mold under sufficient heat and pressure to complete the cure of the plastic material in said fabric and establish a bond between said material and the injected plastic material.

2. The method of molding a heat setting plastic material to make a supporting casing for a resistance element having the form of a compound helix, which consists in providing a mold having a helically grooved core on which the resistance element is supported with the turns thereof in spaced relation, placing a part of said material in the mold along with the core, such material being partially cured and disposed as a covering for the resistance element and the grooved portion of the core, and injecting the remainder of said material into the mold under sufficient heat and pressure to force said covering into continuous engagement with the turns of said helix and the portions of said core which are exposed in the spaces between them.

3. The method of molding a casing around a resistance element'comprising a form wound into a multi-turn helix, said form having a resistance wire wound thereon in spaced turns, which consists in providing a mold with a 'h'elically grooved core to support said helix with'the turns thereof in spaced relation, applying a; covering to said core and helix comprising a layer of partially cured heat setting plastic materia'l reiniorced by a textile fabric, inserting the covered core and helix in the mold, and injecting a heat setting plastic material into the mold under suflicient heat and pressure to form said casing and to convert said covering layer into a lining for the casing in which the turns'of said helix and the turns of said resistance wire are embedded.

4. The method of molding a casing around a resistance element having the form of acompound helix, which consists in providing a mold having a 'helically grooved core, supporting said resistance element on said core, covering said resistance element and the grooved portion of said core with a layer of reinforcing material impregnated with' heat setting plastic material, heatingsaid layer to partially cure said plastic material, inserting said core into said mold, and injecting heat setting plastic material intothe mold under heat and pressure.

'5. Themethod of ino'lding a casing around a resistance elementhaving a support formed into a multi-turn helix, said support having a bare resistance wire helically wound thereon in spaced turns, which consists in providing a mold having a helically grooved ,core, screwing said helix on said core, covering said" helix and the grooved portion of said core with reinforcing material adapted to block the spaces between the turns of said resistance wire against the flow of plastic material into the grooves of said core, inserting said core into said mold, and injecting plastic material into the mold under heat and pressure.

6. Apparatus for molding a casing around a resistance element having the form of a compound helix, comprising a mold having a cavity therein, a core adapted to be inserted into said cavity, said core comprising a cylindrical section having a helical groove for supporting said resistance element, said core including also a section of reduced diameter forming an annular shoulder at the end of said cylindrical section, and said shoulder having the form of a plane which is inclined circumferentially of the core with approximately the same slope as the slope of the said groove, whereby one end of the plane is displaced from the other end axially of the core, and a relatively steep abutment connecting said ends.

7. The method of molding a casing around a resistance element comprising an insulating support wound into a multi-turn helix, said support having a resistance wire wound thereon in spaced turns, which consists in providing a mold with a helically grooved core on which the resistance element is supported with the helix turns in spaced relation, inserting said core and resistance element into said mold, injecting plastic material into said mold under heat and pressure, and blocking the flow of plastic material into the groove of said core through the spaces between the turns of said wire by means of material whichis disposed around the resistance element as a covering before the core is inserted into the mold.

8. Apparatus for molding a casing around a resistance element which comprises a bare resistance Wire Wound on a resilient support, said support being formed into a helix having a greater pitch than the said helix, whereby the' turns of the helix are separated when the resistance element is screwed on to said section prior to insertion of the core in the mold said core section having an overall diameter which is less than the diameter of said cavity, whereby said helical rib is spaced away from the wall of said cavity, the height of said rib being less than the diameter of said support, whereby the helix turns lie partly between adjacent turns of the helical riband partly in the space between the core and the inside wall of said cavity but out of contact with said wall, means for closing the mold at the other. end, and means for injecting plastic material into said cavity under heat and pressure.

9. The method of molding a casing around a resistance element having the form of a compound helix, which consists in providing a mold having a helically grooved core, supporting said resistance element on said core, covering said resistance element and the grooved portion of the core with a layer of textile material impregnated with a heat setting plastic material, heating said fabric after its application to said element and core to partially cure said plastic material, inserting the core in the mold, injecting heat setting plastic material into the mold, and completing the cure of the plastic material in said fabric by applying heat and pressure to the,

injected plastic material.

10. The method of molding a casing around a resistance element comprising an insulating support wound into a multi-turn helix, said support having a bare resistance'wire wound thereon in spaced turns, which consists in providing a mold with a helically grooved core on which the resistance element is supported with the helix turns in spaced relation, inserting said core and resistance element into said mold, injecting plastic material into said mold under heat and pressure, and preventing the injected plastic material from insulating those portions of the turns of the resistance wire which are located in the groove of said core by covering the resistance element with c a plastic impregnated fabric prior to the insertion of the core and resistance element into the mold.

THOMAS B. GIBBS.

, GEORGE W. GILMAN.

GORDON F. LAING.

REFERENCES CITED The following references are of record in the file of this patent:

. UNITED STATES PATENTS Number Name Date 1,315,365 Hamm Sept. 9, 1919 1,630,874 Tuska May 31, 1927 1,982,279 Apple Nov. 27, 1934 2,095,705 Kessler Oct. 12, 1937 2,323,286 Ward June. 29, 1943. 

